The thickness and porosity or permeability of web products such as paper and plastics are important product parameters. For example, lithium-ion battery separators, which are made of porous polymeric membranes, must have uniform thickness (typically to less than one micron) and meet specific electrolyte permeability standards. The art is desirous of developing reliable on-line techniques for measuring the thickness and calculating the permeability of web materials during production.